1. Field of the Invention
The present invention relates to a speaker apparatus used for a television receiver (TV), and more particularly to a speaker apparatus that comprises a microphone for detecting a reproduced sound from a speaker unit and corrects the reproduced sound based on the detected signal.
2. Description of the Related Art
It is known that a speaker apparatus having the following structure contributes to the improvement of acoustic characteristics. A horn or an acoustic pipe whose opening is rectangular is mounted in front of a speaker unit, and a sound wave generated in the speaker unit is guided to the opening of the acoustic pipe. A microphone is mounted in this acoustic pipe and is connected to an amplifier for inputting an input signal into the speaker unit through a feedback circuit.
The prior art discussed above is shown in FIG. 8 and FIG. 9. FIG. 8 is a horizontal sectional view of a conventional acoustic pipe type speaker apparatus with a sound feedback system, and FIG. 9 shows acoustic output characteristics thereof.
In FIG. 8, a speaker unit 1 produces sound wave and is connected with an acoustic pipe 2. Sound absorbing material 3 is disposed for damping resonance on both sides of the acoustic pipe 2. In the acoustic pipe 2, a microphone 4 for detecting an acoustic output signal is placed near the speaker unit 1. When a signal is fed into the speaker unit 1, the speaker unit 1 radiates an acoustic output, and the acoustic output is lead through the acoustic pipe 2 and radiated out from the opening of acoustic pipe 2.
At this time, for preventing a speaker apparatus from having a reproduced-sound-pressure frequency characteristic with radical peaks and dips caused by a standing wave occurring inside the acoustic pipe 2 or a standing wave due to the length of acoustic pipe 2, such standing waves must be damped by the sound absorbing material 3. However, this countermeasure is insufficient, and therefore, microphone 4 detects the acoustic output, i.e. the unrestrainable standing waves, and feeds them back to an amplifier that input a signal into the speaker unit 1. The standing waves occurring in the acoustic pipe 2 are thus damped so that a flat reproduced sound pressure frequency characteristic is obtained.
Frequency characteristics of the speaker unit 1 and the acoustic pipe 2 can be corrected by placing the microphone 4 in front of and close to the speaker unit 1. The characteristic of the acoustic pipe 2 can be corrected by placing the microphone 4 at a position where the sound pressure of a primary resonance of the acoustic pipe 2 is maximum, i.e. at a position of one third of the length of the acoustic pipe 2. The characteristic can be controlled from a low frequency region to the primary resonance region of the acoustic pipe 2 by placing the microphone 4 near the terminal of acoustic pipe 2.
The conventional speaker apparatus discussed above hardly keeps a sufficient oscillation margin, because the microphone 4 detects acoustic outputs of second and higher resonance generated in the acoustic pipe 2, and the microphone 4 also detects a resonance occurring in a closed space which is orthogonal to the longitudinal direction of the acoustic pipe 2, and feeds them back to the amplifier. In addition, the shape of the acoustic pipe 2 becomes complicated for damping the standing wave, and the speaker apparatus becomes expensive due to the use of sound absorbing material 3 or the like.
The present invention aims to address these problems, and provides a speaker apparatus that has a simply structured acoustic pipe and has a stable acoustic characteristic.